src: fix timing of snapshot serialize callback

Previously the addAfterUserSerailizeCallback() wasn't
ready to be used for building the built-in snapshot.
This patch initializes the callbacks at the time
lib/internal/v8/start_snapshot.js is loaded, so that
these callbacks get run correctly when building the
built-in snapshot.

Currently when building the built-in snapshot,
addAfterUserSerializeCallback() is only used by createUnsafeBuffer(),
other usages can only come from user-land snapshots,
which is covered by tests, but what gets run by the
built-in snapshot building process is less visible, and the
path used by createUnsafeBuffer() isn't reliably visible in user
land either. This adds an internal usage counter in debug builds
to verify this path when building the built-in snapshot.

Author: joyeecheung

lib/internal/main/mksnapshot.js

@@ -21,7 +21,6 @@ const { emitExperimentalWarning } = require('internal/util');
 const { emitWarningSync } = require('internal/process/warning');
 
 const {
-  initializeCallbacks,
   namespace: {
     addDeserializeCallback,
     isBuildingSnapshot,
@@ -139,7 +138,6 @@ function requireForUserSnapshot(id) {
 
 function main() {
   prepareMainThreadExecution(false, false);
-  initializeCallbacks();
 
   // In a context created for building snapshots, V8 does not install Error.stackTraceLimit and as
   // a result, if an error is created during the snapshot building process, error.stack would be

lib/internal/v8/startup_snapshot.js

@@ -115,8 +115,8 @@ function setDeserializeMainFunction(callback, data) {
   });
 }
 
+initializeCallbacks();
 module.exports = {
-  initializeCallbacks,
   runDeserializeCallbacks,
   throwIfBuildingSnapshot,
   // Exposed to require('v8').startupSnapshot

test/parallel/test-buffer-alloc-unsafe-is-uninitialized.js

@@ -0,0 +1,17 @@
+// Flags: --expose-internals
+// Verifies that Buffer.allocUnsafe() indeed allocates uninitialized memory by checking
+// the usage count of the relevant native allocator code path.
+'use strict';
+
+const common = require('../common');
+if (!common.isDebug) {
+  common.skip('Only works in debug mode');
+}
+const { internalBinding } = require('internal/test/binding');
+const { getGenericUsageCount } = internalBinding('debug');
+const assert = require('assert');
+
+const initialCount = getGenericUsageCount('NodeArrayBufferAllocator.Allocate.Uninitialized');
+Buffer.allocUnsafe(Buffer.poolSize + 1);
+const newCount = getGenericUsageCount('NodeArrayBufferAllocator.Allocate.Uninitialized');
+assert.notStrictEqual(newCount, initialCount);